1. Fields of the Invention
The present invention relates to a method for fabricating an aluminum electrode, especially to a method for fabricating a silicon-doped or boron-doped aluminum electrode of a solar cell that is in local contact with a silicon wafer.
2. Descriptions of Related Art
Solar power is among one of the most prosperous renewable energy and solar cell related research has received considerable attention. Solar cells directly convert light energy into electricity by the photovoltaic effect without emission of greenhouse gases and pollutant gases such as carbon dioxide, nitrogen oxides, sulfur oxides, etc. Thus solar energy becomes a promising alternative source as energy resources are gradually being exhausted and the cost of energy is increasing.
However, for long-term and wider applications of solar cells, there are still many technical problems need to be overcome. The unsolved problems of solar cells include stability, lifetime, cost, etc.
Take a crystalline silicon solar cell as an example, surface passivation of the silicon wafer is crucial. Silicon atoms on surface of the silicon wafer have many dangling bonds that promote recombination of carriers. An increase in surface recombination reduces the photovoltaic conversion efficiency. Thus a layer of passivation material is deposited on surface of the crystalline silicon solar cell to form a passivation layer. Thereby surface defects are reduced to ensure minimal surface recombination losses and further improve energy conversion efficiency. Yet most of the passivation layer is made from insulating dielectric material. While producing contact electrodes, a metal layer covered over surface of the passivation layer is processed to pass through the passivation layer and have ohmic contact with silicon wafer. Thus a circuit is formed.
There are three methods commonly used now. The first method is using photolithography to form openings on the passivation layer. Then an aluminum electrode is deposited by screen printing or application of aluminum paste. This is the most mature method now but the processes are complicated and mostly used in laboratories, not suitable for mass production.
The second method is laser ablation. There are multiple openings formed by using a laser beam to irradiate the passivation layer and then deposit an aluminum electrode or screen print aluminum paste. Yet the method is easy to form holes between aluminum and silicon and this affects electrode contact performance. Moreover, in order to prevent damages to the silicon wafer, the opening formation process usually uses laser with extremely short wavelength such as excimer laser that produces energy in the ultraviolet spectrum. Within the range of such short wavelength, the laser power is low and the cost is quite high.
The third is to deposit the aluminum layer over the passivation layer by laser firing contact. Then both the aluminum layer and the passivation layer are penetrated by a laser beam at the same time to form ohmic contact between aluminum and silicon. The method has two shortcomings. (1) The electrode contact of aluminum silicon alloy is obtained by high temperature of the laser. Besides the speed problem, the laser processing also causes damages to the silicon wafer around the processed area of the silicon wafer. (2) The surface electric field is difficult to form around the aluminum electrode due to low aluminum solid solubility in silicon. Thus carrier recombination near the aluminum electrode is unable to be reduced efficiently and the conversion efficiency of the cell is further affected.
There is room for improvement and a need to provide a novel method for fabricating an aluminum electrode that achieves a good balance between the cost and the performance.